24 comments:

I am the first to admit I am not a circuits guy so maybe that is why I am having trouble understanding this pixel circuit and how it is all biased. Anyway, I am sure the these guys have this all figured out so I am hoping they can explain more details here. Maybe a paper at IISW?e.g. what does D1 do relative to the Q1 E-B diode?I thought a p-gate JFET J1 should have its gate reversed biased relative to S and D? (I understand you can get away with small positive bias).How does this circuit not draw dc current?What is the time response of this circuit

I like the idea of using JFETs and bipolars to reduce circuit noise (e.g. RTS perhaps) but I am not sure this works well for, say, 100 e-/sec current in the detector.

<< Funny information! Just having taken a look this guy's CV. How can a software engineer invent soundable pixel design and state that the noise can be eliminated? >>

This is poor, unconstrutive comment that Image Sensors should remove. People from different backgrounds have different experiences and perspectives on approaching problem solving - This is the key to innovation and creativity.

I'm not going to remove it. However, I agree with you that good ideas can come from everybody, no matter what his/her background is. The only difference is the frequency: professionals in the field tend to generate good ideas much more frequently, in my practice, anyway.

In any case, I would not reject any idea based on the originator's background.

if you claim 200dB dynamic range, you have to decrease at least several orders of magnitude the noise floor. It's necessary that this gentleman show the details to image sensor community this invention.

@ "It has always seemed obvious to me that a sensor could be using photo-resistors and be measuring resistance."

Photoresistors usually have higher noise than photodiodes, not to say about dark current and nonuniformity. Photodiodes in photovoltaic mode also have uniformity challenge and their noise is somewhat higher.

@ "if you claim 200dB dynamic range, you have to decrease at least several orders of magnitude the noise floor."

Since IsInvariant's PD does not have reset, I'd speculate that it's effective integration time varies depending on illumination. At low light the integration time goes up, so the SNR degradation is less obvious. However, longer integration can be interpreted as a lag. Also, the PD should be of very high quality, virtually defect-free to extend sensitivity to a significant degree.

For normal CMOS sensor after integration the accumulated image charge is stored for conversion to digital numbers. The theoretical dynamic range is ratio of the number of full-well electrons to that of the noise electrons. However, if you check any CMOS sensor datasheet form the major suppliers, they never show the number of noise electrons. Because it is the most difficult part to control. The next problem is to get a good, say, 16-bit ADC with minimum additional noise. None of the avove problems are easy to solve. The wide dynamic range CMOS sensor MT9M033 released by Aptina recently relys on two or three exposures with different shutter time to achieve wider dynamic range as high as 120dB. It tells us something.

1. The read noise comes mostly from 1/f noise in the output transistor. Probably a JFET would be better in the output stage.2. right3. Just like every other CMOS image sensor pixel.4. I hope so5. How will you avoid kTC noise on reset? I guess the effective input photon noise is worse for a logarithmic pixel.6. ok7. Indeed. In photovoltaic mode you can get by with the gate slightly positive relative to the source.8. Photovoltaic mode is of course not new. A big problem with this kind of pixel (besides those already pointed out previously) is that images taken under low contrast (e.g. cloudy conditions outdoors) don't come out well when companded by the logarithmic-response device. And, if you try to stretch the contrast, all the non-uniformity warts come out to bite you on the butt.

With the modifications described, it will be difficult to patent this pixel due to prior art. In my opinion the commercial viability of this approach is not as rosy as you may think because there is no new nor compelling advantage to this approach.

Nevertheless, best of luck. I think the money for you may be in the system, not in the pixel.

I should add that I think the device physics at a few hundred electrons per second incident on the photovoltaic device might be interesting. Probably equivalent circuit models are not too good for the time-domain for this device due to low signal statistics. That is, current flow in the photodiode may have a high shot-noise component. Maybe the low background IR astronomy guys have looked at this...

Wouldn't it be nice if a ring-geometry JFET worked out well, so you could use the ring gate as detector (PV mode)? Possibly patentable (at least up until now) but it has all the issues described above not to mention the pesky microlens issue that Olympus encountered with its CMD device.